A ball joint for movably connecting shaft-like components to each other is conventionally used in suspensions and steering mechanisms for vehicles such as automobiles. A ball joint is primarily constituted such that a spherical ball portion which is formed on the end portion of a shaft-like stud is slidably received in a cylindrical bearing seat which is held inside a cast housing. The bearing seat is incorporated into the housing as a result of the molding of the housing by previously disposing it as a core inside a mold at the time of casting (die casting) of the housing.
With such a so-called integral-type ball joint, the coefficient of contraction of the material forming the housing (such as an aluminum alloy, magnesium, or zinc) is different from the coefficient of contraction of the material forming the bearing seat (such as a PEEK resin). Therefore, due to gaps which form between the molded housing and the bearing seat, play and looseness develop between the two, and there is a concern of a decrease in the stiffness of the ball joint. Therefore, below-described Patent Document 1, for example, discloses a ball joint which can suppress play and looseness between a housing and a bearing seat by providing undercut grooves in the contact surface between the housing and the bearing seat.